1. Field of the Invention
This invention relates to a nematic liquid crystal composition. More particularly, it relates to a nematic liquid crystal composition which, when used for liquid crystal display elements, improves the temperature dependency of the threshold voltage thereof and also improves the temperature dependency of the intrinsic helical pitch (hereinafter referred to as "P").
2. Description of the Prior Art
As to TN type liquid crystal display elements, their use applications have been rapidly enlarged due to improvements in circuit, driving mode and cell preparation technique and particularly improvement in the characteristics of liquid crystal compositions sealed in the elements, although their use applications had been directed only to watches, electric calculators, etc. in their initial use.
Such a rapid enlargement of the use applications due to improvements in liquid crystal compositions and others owes a great deal to .circle.1 increase in the display capacity and .circle.2 broadening of the temperature range of the nematic liquid crystal phase.
With respect to the increase in the display capacity, displays of hand-held computors and liquid crystal television are illustrated as use applications. With respect to the broadening of the temperature range of nematic liquid crystal phase, displays for instruments used on cars and instruments for outdoor use are illustrated. However, there are a large number of matters to be improved for liquid crystal display elements. Examples of such matters are narrow angle of view, inferior contrast, low response speed, still yet small display capacity, reduction in the display quality due to ambient temperature change, etc. Among these, the reduction in the display quality due to ambient temperature change is attributed to the change in the threshold voltage V.sub.th depending on temperature.
In order to suppress the occurrence of the reverse twist of liquid crystal molecules to thereby retain the display quality of liquid crystal display elements, it has been very usually carried out to add a slight quantity of an optically active substance having a clockwise or counterclockwise helical twist sense. However, there is a problem that since the threshold voltage of liquid crystal compositions still yet has a considerable temperature dependency, reduction in the display quality due to ambient temperature change is unavoidable.
With respect to improvements in the angle of view and contrast, considerable improvements have been made by employing supertwisted birefringence effect mode (abbreviated to SBE). SBE mode is different in certain points from TN mode. Firstly, according to TN mode, addition of a slight quantity of an optically active substance helps the glass substrates subjected to aligning treatment to twist liquid crystal molecules by 90.degree. between substrates due to their anchoring action. Here, the ratio (P/d) of the intrinsic helical pitch (P) of liquid crystal composition to the cell thickness (d) of display element is usually about 10-20. According to SBE mode, however, by increasing the quantity of an optically active substance added to a large extent to thereby make the value of P/d 2 or less, the liquid crystal molecule is twisted by 270.degree.. Further, according to TN mode, liquid crystal molecules are aligned in a state where no voltage is impressed, inside the display element, so as to give an angle of liquid crystal molecule made against glass substrates (pre-tilt angle) within several degrees, whereas according to SBE mode, alignment is made so as to give a pre-tilt angle of about 20.degree.. An example of having improved the angle of view and contrast according to such SBE mode has been reported (T. J. Scheffer, J. Nerhring, M. Kaufmann, H. Amstutz, D. Heimgartner and P. Eglin, Society for Information Display, 1985, International Symposium).
However, this SBE mode, too, raises a problem. Namely, since the intrinsic helical pitch P varies depending on the temperature change, it occurs that when the value of P/d exceeds 2, 170.degree. twist is changed to 90.degree. twist; hence it is necessary to keep the intrinsic helical pitch P at a constant value irrespective of temperature.
Further, with respect to improvement in the display capacity, it is necessary to improve the steepness of change in the transmittance in the case where a voltage is impressed to the display element. G. Bauer and W. Fehrenbach reported a calculation result that the steepness is improved to a large extent at 270.degree. twist (15. Freiburger Arbeitstagung Flussigkristalle (1985)). In this case, too, it is necessary to be free from change in the intrinsic helical pitch depending on the temperature change.
With respect to the improvement in the response speed, Nakagawa and Masuda reported that response speed has been improved by employing a double-layered guest-host mode (abbreviated to DGH mode) wherein two pieces of a liquid crystal display element of guest-host type are placed on one another and liquid crystal compositions having P/d=1.0 (i.e. 360.degree. twist) are employed. (Nakagawa and Masuda, Society for Information Display, 1985 International Symposium). In this case, too, it is important to be free from change in the intrinsic helical pitch depending on the temperature change.
Further, in the display elements of the phase transition mode (PC mode), too, it is better to be free from change in the intrinsic helical pitch depending on the temperature change. Further, with respect to overcoming reduction in the display quality depending on ambient temperature change, this may be effected by reducing the temperature dependency of the threshold voltage V.sub.th.
As to the cause of change in the threshold voltage V.sub.th depending on the temperature range, changes in the elastic constant of nematic liquid crystals, the dielectric anisotropy, etc. depending on the temperature change, change in the intrinsic helical pitch depending on the temperature change, etc. are enumerated. In order to improve the temperature dependency of the threshold voltage, certain attempts have been made, and among these, a process of improving the temperature dependency of the threshold voltage by controlling change in the intrinsic helical pitch depending on the temperature change has often been carried out.
When an optically active substance is added to a nematic liquid crystal, there is the following equation (1) between the concentration C of the optically active substance and the intrinsic helical pitch P of the resulting liquid crystal composition, and in addition, the reciprocal of the intrinsic helical pitch P.sup.-1 is also referred to as "twistability" and exhibits the strength of twist: EQU P.sup.-1 =h.multidot.C (1)
wherein h is referred to as helical twisting power and a constant intrinsic of the optically active substance and varies depending on temperature. The change of h depending on the temperature change is expressed by the following equation (2): EQU h=.alpha.+.beta.T+.gamma.T.sup.2 + - - - (2)
wherein .alpha., .beta., .gamma., - - - each represent a proportionality factor.
An example of the twistability (P.sup.-1) dependency of the threshold voltage V.sub.th in the case where the temperature is constant and also the cell thickness of the TN type liquid crystal element is constant, is shown in FIG. 1. FIG. 1 shows the relationship between the twistability (P.sup.-1) and the threshold voltage V.sub.th in the case where an optically active substance C-1 expressed by ##STR3## is added to a nematic liquid composition A shown below: ##STR4##
As seen from FIG. 1, the threshold voltage V.sub.th rises with an increase of the twistability (P.sup.-1). Namely, the longer the intrinsic helical pitch P of the liquid crystal composition becomes, the more the threshold voltage V.sub.th is reduced.
Further, the temperature dependency of the twistability (P.sup.-1) in the case where the optically active substance C-1 is added in 0.4% by weight to the above nematic liquid crystal composition A is shown in FIG. 2. As seen from FIG. 2, the twistability (P.sup.-1) decreases with temperature rise, and the intrinsic helical pitch P of the liquid crystal composition increases with temperature rise.
On the other hand, the temperature dependency of the threshold voltage V.sub.th is shown in FIG. 3. The threshold voltage V.sub.th lowers with temperature rise. This indicates that, as seen from FIG. 1 and FIG. 2, the intrinsic helical pitch P of the liquid crystal composition increases with temperature increase to thereby lower the threshold voltage V.sub.th. Further, it has been known that the threshold voltage V.sub.th lowers depending on decrease in the elastic constant of the composition due to the temperature rise.
Thus, in order to reduce the temperature dependency of the threshold voltage V.sub.th, the intrinsic helical pitch P of the liquid crystal composition is preferred to be shorter with the temperature rise.
As apparent from the foregoing, control of the temperature dependency of the intrinsic helical pitch is very important for overcoming various problems raised on the liquid crystal display elements of various display modes. Namely, as to SBE mode, DGH mode and PC mode, the intrinsic helical pitch has been required to be constant irrespective of temperature. Further, in order to improve the temperature dependency of the threshold voltage in the case of TN mode, the intrinsic helical pitch has been required to be shorter with the temperature rise. However, too steep reduction in the cholesteric pitch with temperature rise is not always satisfactory; thus it is also necessary to adjust the extent of the change of the intrinsic helical pitch depending on the temperature change. However, if a generally known optically active substance is added, the intrinsic helical pitch of the resulting nematic liquid crystal composition increases with the temperature rise. In short, the twistability (P.sup.-1) decreases with temperature rise; hence, even if the substance alone is added, it is impossible to control the temperature dependency of the intrinsic helical pitch. That is, it is impossible to be free from the temperature dependency of the intrinsic helical pitch or to obtain a temperature dependency which is contrary to the conventional one.
When a plurality of optically active substances are added to nematic liquid crystals, the intrinsic helical pitch P.sub.Mix of the resulting liquid crystal composition is expressed by the following equation (3): ##EQU1##
This equation (3) indicates that the P.sub.Mix.sup.-1 of the final liquid crystal composition is the sum of the respective P.sub.i.sup.-1 s obtained when the respective optically active substances are singly added to the original nematic liquid crystals in a concentration of C.sub.i.
In addition, when the symbol h of helical twisting power is made positive relative to right-twisted optically active substance and made negative relative to left-twisted optically active substance, the intrinsic helical pitch P.sub.Mix of the liquid crystal composition obtained by adding the mixture of right-twisted and left-twisted optically active substances to nematic liquid crystals is also expressed by the equation (3).
In the case of conventional optically active substances, even if two optically active substances each having a twist in the same sense are mixed and the mixture is added to nematic liquid crystals, the resulting temperature dependency of the intrinsic helical pitch is nothing but an intermediate one between the two dependencies derived from the respective optically active substances; thus it is impossible to be free from the temperature dependency or to obtain a temperature dependency which is contrary to the conventional one. Now, it has been reported that when an optically active substance having a helical twist right sense is mixed with that having a helical twist left sense in a definite proportion and the mixture is added to nematic liquid crystals, then it is possible to be free from the temperature dependency of the intrinsic helical pitch or to obtain a temperature dependency which is contrary to the conventional one (e.g. see U.S. Pat. No. 4,264,148, issued Apr. 28, 1981). In this case, however, an optically active substance having right twist and that having left twist are mixed so as to compensate these twists relative to one another to thereby obtain a definite intrinsic helical pitch; hence there is a case where the twistability (P.sup.-1) becomes zero even in the vicinity of room temperature, depending on the mixing proportions, and above and below this temperature, the twisting senses are reverse to one another to thereby notably reduce the display quality of liquid crystal display elements using a liquid crystal composition of this type. Thus, only a considerably limited range of the mixing proportion will be employed. Further since the change of the intrinsic helical pitch is notable due to a slight difference of the mixing proportion, the temperature control of the intrinsic helical pitch is considerably difficult.
Further, since both a right twist optically active substance and a left twist one are added, one cannot help increasing the quantities thereof added, in order to obtain a desired helical pitch. Thus, the characteristics of the resulting nematic liquid crystal composition such as transition point, viscosity, threshold voltage V.sub.th, etc. change considerably from the characteristics of the original nematic liquid crystals. Further, since optically active substances are expensive, the final liquid crystal composition is also expensive. On account of these drawbacks, the practical use of a liquid crystal composition having added such two kinds of right twist and left twist optically active substances has been notably restricted.